In the fields of fertilizers such as ammonium sulfate fertilizers and urea fertilizers, various chemical products, pharmaceutical products and food products, various crystallization apparatuses are used for producing crystalline particles. From the standpoint of the type of heating, the crystallization apparatuses can be classified into crystallization apparatuses of the inner heating type and crystallization apparatuses of the outer heating type. From the standpoint of stirring, the crystallization apparatuses can be classified into crystallization apparatuses of the spontaneous stirring type using boiling of a liquid and crystallization apparatuses of the forced stirring type having stirring blades disposed inside the apparatuses.
It is important for every type of the crystallization apparatus that the average particle diameter and the distribution of the particle diameter of crystals are controlled and sticking of crystals to the wall surfaces of the crystallization apparatuses is prevented. Various attempts have been made to control the average particle diameter and the distribution of the particles diameter and to prevent sticking of crystals to the wall surfaces of the apparatuses.
In the crystallization apparatuses, the retention time of a slurry in the crystallization (the volume of a crystallization tank) is decided from the material balance and the time of crystallization necessary for growth of crystals (the residence time of crystals) when the condition of the crystallization is decided. When the average particle diameter or the distribution of the particle diameter as the target value is changed, the new target value must be achieved by a change in the retention time of a slurry during the crystallization within the range which the change in the liquid level in the crystallization tank allows or by changes in the temperature and the pressure of the crystallization. Naturally, the range of the change in the above conditions is restricted.
To overcome the above restriction, various attempts have been made for a long time. For example, a complicated structure is formed inside the crystallization tank. Typical examples of such structures include the crystallization tank of the Oslo type (Crystal Oslo) and the crystallization tank of the draft tube type. For example, in a process described in Japanese Patent Application Laid-Open No. Heisei 6(1994)-226003, an annular tube having many outlets for blowing a liquid is disposed at a lower end of an outer tube of a draft tube, an outer circulating apparatus is connected to the annular tube and a solution is supplied to a crystallization tank via the annular tube.
To prevent sticking of crystals, local measures are taken at portions where crystals stick and grow as described in the following.
1) To suppress sticking of crystals at liquid surface portions of an apparatus, the surface portions are treated by various polishing (buffing or electrolytic polishing) or coating with a material suppressing sticking (such as coating with Teflon).
2) To provide the solution in the vicinity of liquid surface portions of an apparatus with a property of preventing forming crystals, the solution in the vicinity of liquid surface portions of the apparatus is substituted or washed with a solution having a concentration lower than the saturation, heated or cooled.
3) To prevent growth of crystals even when crystals are stuck, the crystals are mechanically removed.(removed by scraping blades).
For example, in a process described in Japanese Patent Application Laid-Open No. Heisei 10(1998)-156103, a solvent is vaporized from a super saturated solution containing a crude material for crystallization under a reduced pressure while the solution is stirred by a stirrer having stirring shaft, the vaporized solvent is condensed at the outside of a crystallization tank and the condensed solvent is sprayed to the gas phase portion of the crystallization tank so that sticking of crystals at the gas phase portion of the crystallization tank is prevented.
As described above, various attempts have been made on the crystallization tank to improve the distribution of the particle diameter and prevent sticking of crystals to walls. However, in general, complicated apparatuses or operations are required and practical effects are not achieved in most cases. For example, in the process described in Japanese Patent Application Laid-Open No. Heisei 10(1998)-156103, the solvent obtained by the vaporization must be sprayed to the inside of the crystallization tank. The concentration of the material in the crystallization tank which is already concentrated is diluted again with the sprayed solvent and the efficiency of the concentration decreases. In the process described in Japanese Patent Application Laid-Open No. Heisei 6(1996)-226003, a complicated tube having outlets for blowing a liquid is required.